$ F = \left[\begin{array}{rr}4 & 1 \\ -2 & -2\end{array}\right]$ $ D = \left[\begin{array}{rrr}-1 & 4 & 4 \\ 2 & 1 & 4\end{array}\right]$ What is $ F D$ ?
Solution: Because $ F$ has dimensions $(2\times2)$ and $ D$ has dimensions $(2\times3)$ , the answer matrix will have dimensions $(2\times3)$ $ F D = \left[\begin{array}{rr}{4} & {1} \\ {-2} & {-2}\end{array}\right] \left[\begin{array}{rrr}{-1} & \color{#DF0030}{4} & \color{#9D38BD}{4} \\ {2} & \color{#DF0030}{1} & \color{#9D38BD}{4}\end{array}\right] = \left[\begin{array}{rrr}? & ? & ? \\ ? & ? & ?\end{array}\right] $ To find the element at any row $i$ , column $j$ of the answer matrix, multiply the elements in row $i$ of the first matrix, $ F$ , with the corresponding elements in column $j$ of the second matrix, $ D$ , and add the products together. So, to find the element at row 1, column 1 of the answer matrix, multiply the first element in ${\text{row }1}$ of $ F$ with the first element in ${\text{column }1}$ of $ D$ , then multiply the second element in ${\text{row }1}$ of $ F$ with the second element in ${\text{column }1}$ of $ D$ , and so on. Add the products together. $ \left[\begin{array}{rrr}{4}\cdot{-1}+{1}\cdot{2} & ? & ? \\ ? & ? & ?\end{array}\right] $ Likewise, to find the element at row 2, column 1 of the answer matrix, multiply the elements in ${\text{row }2}$ of $ F$ with the corresponding elements in ${\text{column }1}$ of $ D$ and add the products together. $ \left[\begin{array}{rrr}{4}\cdot{-1}+{1}\cdot{2} & ? & ? \\ {-2}\cdot{-1}+{-2}\cdot{2} & ? & ?\end{array}\right] $ Likewise, to find the element at row 1, column 2 of the answer matrix, multiply the elements in ${\text{row }1}$ of $ F$ with the corresponding elements in $\color{#DF0030}{\text{column }2}$ of $ D$ and add the products together. $ \left[\begin{array}{rrr}{4}\cdot{-1}+{1}\cdot{2} & {4}\cdot\color{#DF0030}{4}+{1}\cdot\color{#DF0030}{1} & ? \\ {-2}\cdot{-1}+{-2}\cdot{2} & ? & ?\end{array}\right] $ Fill out the rest: $ \left[\begin{array}{rrr}{4}\cdot{-1}+{1}\cdot{2} & {4}\cdot\color{#DF0030}{4}+{1}\cdot\color{#DF0030}{1} & {4}\cdot\color{#9D38BD}{4}+{1}\cdot\color{#9D38BD}{4} \\ {-2}\cdot{-1}+{-2}\cdot{2} & {-2}\cdot\color{#DF0030}{4}+{-2}\cdot\color{#DF0030}{1} & {-2}\cdot\color{#9D38BD}{4}+{-2}\cdot\color{#9D38BD}{4}\end{array}\right] $ After simplifying, we end up with: $ \left[\begin{array}{rrr}-2 & 17 & 20 \\ -2 & -10 & -16\end{array}\right] $